Intra-atrial reentrant tachycardia is a well recognized sequelae of the Fontan operation. The incidence of both early and late intra-atrial reentrant tachycardia is 10-35%. (1-5) Data from animal models as well as studies in humans in the clinical electrophysiology laboratory suggest that the suture line of the lateral cavopulmonary connection and the natural anatomical right atrial boundaries (vena cava orifices, coronary sinus orifice, tricuspid valve, and possibly the crista terminalis) can serve as barriers creating a pathway which, along with changes in intra-atrial conduction, provide the substrate for an intra-atrial reentrant tachycardia. (6-12) The objective of this study is to investigate if a strategically placed linear incision, incorporated at the completion of the fenestrated Fontan, will prevent the postoperative intra-atrial arrhythmia. The study is a randomized prospective design in which candidate patients will receive either the conventional fenestrated Fontan repair (control group), or a fenestrated Fontan repair with a cryosurgery lesion or surgical incision (test group). The primary endpoint will be based upon the failure to induce the intra-atrial reentrant tachycardia in the test group compared to the control group both during the immediate post operative period (prior to hospital discharge) and at the time of fenestration closure (6-12 months after surgery). The patient groups will also be followed for a five year period to determine the effect of the surgical incisional lesion on the prevention of late clinical reentrant intra-atrial tachycardia.